The invention relates to inductive sensors of planar type, which allow detection of the discontinuities of a conductive or ferromagnetic part (or target) without contact therewith.
Magnetic induction sensors of planar type are known and are described in European Patent Applications No. 97 106 884.6 and No. 98 400 835.9 for example. Such a miniaturized inductive sensor comprises at least one primary winding traversed by an alternating current which creates an alternating magnetic field and at least one secondary winding subject to this alternating field, the windings being planar. The amplitude and the phase of the magnetic flux passing through each secondary winding are modified by the passage of a metallic or ferromagnetic part (or target) having at least one discontinuity, such that the passage of this discontinuity is detected by measuring the amplitude or phase modulation of the electric voltages induced in the secondary windings, which allows determination of the type of discontinuity and the sense and speed of the movement of the metallic part through suitable processing of the signals representing these voltages.
Such sensors are used in many fields and, particularly in the field of automobiles, to detect the speed of rotation of the engine shaft or of wheels, by means of a toothed wheel for example, whose teeth pass in front of the inductive sensor, which creates variations in the voltage induced in the secondary windings during the passage of the flanks of the teeth. The amplitude of the variations depends strongly on the position and orientation of the sensor relative to the target. Thus the geometry of the coils and the mounting of the sensor are adapted to the shape of the teeth, or marks, of the target.
In most applications, the inductive sensor has to be located with precision relative to the target, so as to obtain a satisfactory electric signal, for example with as high as possible a signal-to-noise ratio, to ensure good detection even in the presence of noise introduced by the elements used to process the signal. Such precise positioning of the sensor relative to the target is difficult to effect however, or holding its position with the passage of time cannot always be ensured and the initial adjustment then gets modified.
One object of the present invention is thus to provide an inductive sensor which allows the signal-to-noise ratio to be optimized, even in the event of non-optimal positioning of the sensor relative to the target.
Another object of the present invention is also to provide an inductive sensor whose positioning relative to the target is simpler, cheaper, more rapid, more reliable and thus less troublesome.
A further object of the invention is to allow the use of a large number of receiving coils whose output signals are processed in an overall manner in such a way as to allow information to be recovered, for example on the shape of the discontinuity of the target (magnetic image), or the position or the movement of a predetermined shape.
The invention thus provides an optimized inductive sensor of the type comprising a primary winding fed with an alternating current supplied by an excitation device and multiple secondary windings subject co the magnetic field created by the primary winding and providing electric signals at the output terminals representative of the variations in the magnetic field due to the presence of a metallic target with discontinuities, the said electric signals being applied to an amplification-demodulation circuit followed by a processing circuit for the demodulated signals, characterized in that:
the secondary windings have minimum dimensions compatible with detection of the signals induced by the variations in the magnetic field, in such a manner as to be able to associate a maximum number of secondary winding with a primary winding between two discontinuities of the target, and
the output terminals of the secondary windings are connected to interconnection means which effect adaptive connection of the said output terminals in such a manner as to connect the secondary windings as a function of the discontinuities of the target, in a stored form or as a desired transfer function.
The optimized sensor of the invention can advantageously be associated with an electronic interface circuit through which one or more of the following functions can be executed:
local or global analysis or analysis according to given groupings of the signals provided by the secondary windings;
grouping of the signals provided by the secondary windings according to a defined representation;
grouping of the signals provided by the secondary windings according to a configuration determined by the interface circuit itself to apply a specified transfer function.
In the case of adaptation to the marks of the target, couples of secondary windings are grouped by the said interconnection means to obtain signals whose signal-to-noise ratio is as good as possible.
The interconnection means can be realized by metallic conductor elements, by electronic switches whose opening and closing are effected through electric signals provided by an adaptive circuit. The interconnections can also be determined by an auto-calibration procedure. They can for example be realized by means of predefined metal connections or miniature electronic switches, whose closing depends on the desired regrouping, or on the local or global transfer function which it is desired to implement. Moreover, it is possible to add a calculation and/or weighting function to all or a part of the secondary windings.